Sky Survey creates 3D map of the universe that stretches back seven billion years
A massive collaborative effort between teams of U.S. scientists has produced the largest ever 3D map of the cosmos.
The Luminous Galaxies map has produced so much data that it has allowed measurements of the universe - which is constantly expanding - that stretch back seven billion years in time
By measuring the light and colour of almost a million galaxies - ranging across the largest volume of space ever used for such measurements - the teams have been able to construct a series of new measurement charts for the universe.
The Luminous Galaxies chart is the biggest ever 3D map of the universe. Here, 'z' is the point where the universe was equally made up of photons and matter, and the numbers are the distance from that point
Since 2000, over the course of three major surveys, the Sloan Digital Sky Survey (SDSS) team has measured 4,000 square degrees of the sky - more than a third of its total area - and delivered over a trillion pixels of imaging data for the map.
The SDSS team worked with colleagues from the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) and the DOE's National Energy Research Scientific Computing Center (NERSC).
The information they gathered has allowed them to produce the most accurate calculation yet of how matter clumps together – from a time when the universe was only half its present age (roughly 13.7billion years old) until now.
The volume of data was such that it took 60,000 hours of computer processing just to get it ready for analysis.
'The way galaxies cluster together over vast expanses of the sky tells us how both ordinary visible matter and underlying invisible dark matter are distributed, across space and back in time,' says Shirley Ho, an astrophysicist at the U.S. Department of Energy's Berkeley Laboratory.
'The distribution gives us cosmic rulers to measure how the universe has expanded, and a basis for calculating what's in it: how much dark matter, how much dark energy, even the mass of the hard-to-see neutrinos it contains.
'What's left over is the ordinary matter and energy we're familiar with.'
Colour spectrum: Our Milky Way galaxy (blue) is seen here absorbing its galactic neighbor, Sagittarius, (red trail) in 2003; the colour of a galaxy indicates its age
To create the map Ho and her colleagues measured the light and colour produced by galaxies.
They chose 900,000 luminous galaxies from more than 1.5 million candidates recorded by the Baryon Oscillation Spectrographic Survey (BOSS), the largest component of the ongoing SDSS III.
Most of those galaxies are ancient, meaning they are red-coloured and exceptionally bright, so as to be visible at great distances.
Their brightness was measured in five different colors, allowing their 'redshift' - the galaxies' speed and distance from earth - to be estimated.
It has allowed them new insights into the history of the universe, including how far and how fast it has expanded.
'By covering such a large area of sky and working at such large distances, these measurements are able to probe the clustering of galaxies on incredibly vast scales, giving us unprecedented constraints on the expansion history, contents, and evolution of the universe,' says Professor Martin White of BOSS.
'The clustering we're now measuring on the largest scales also contains vital information about the origin of the structure we see in our maps... and it helps us to constrain – or rule out – models of the very early universe.'
Among their most interesting findings are that dark energy accounts for 73 per cent of the density of the universe.
Mapping the sky: These images from the ongoing Sloan Digital Sky Survey III show the northern (left) and southern hemispheres of the Earth, as seen from space, were produced in early 2011
That in turn allowed the team to produce a power spectrum, which shows how varies in density across the sky.
That means information about inflation – how the universe rapidly expanded shortly after the big bang – can be derived from the power spectrum.
Closely related to the power spectrum are two 'standard rulers', which can be used to measure the history of the expansion of the universe.
The first ruler has only a single mark but it is a big one – the starting point when matter and radiation were exactly equal in density in the universe.
'In the very early universe, shortly after the big bang, the universe was hot and dominated by photons (particles of radiation)' said Ho.
'But as it expanded, it began the transition to a universe dominated by matter. By about 50,000 years after the big bang, the density of matter and radiation were equal. Only when matter dominated could structure form.'
The other cosmic ruler is also very big, but it has multiple marks in the power spectrum; this ruler is called BAO, which stands for baryon acoustic oscillations.
Baryon acoustic oscillations are the echoes of sound waves that traveled through the early universe when it was a hot, liquid-like soup of matter and photons.
After about 50,000 years the matter began to dominate, and by about 300,000 years after the big bang the soup was finally cool enough for matter and light to go their separate ways.
Differences in density that the sound waves had created in the hot soup, however, left their signatures as statistical variations in the distribution of light, detectable as temperature variations in the cosmic microwave background (CMB), and in the distribution of baryons.
The CMB is a kind of snapshot of radiation in the universe that can still be read today.
The BAO rule is the target of the Baryon Oscillation Spectroscopic Survey. By the time it's completed, BOSS will have measured the individual spectra of 1.5 million galaxies, a highly precise way of measuring their 'redshifts'.
The first BOSS spectroscopic results are expected to be announced early in 2012.
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